Charge-coupled device (CCD) image sensors and complementary metal-oxide semiconductor (CMOS) image sensors are the two major types of electronic image sensor currently in use. CCD image sensors can provide excellent light sensitivity and high image quality, but manufacturing of CCD image sensors generally requires specialized fabrication processes that make CCD sensors more expensive to make and more difficult to integrate with associated circuitry. CMOS image sensors, on the other hand, can be inexpensively fabricated using standard CMOS manufacturing technology and can be easily integrated on the same die with circuit blocks serving other imaging and non-imaging functions. However, high light sensitivity and high image quality are more difficult to achieve with CMOS image sensors.
FIG. 1 illustrates a conventional CMOS image sensor 100, which includes an array 110 of pixel sensors 120. Control lines (e.g., row lines 112 and column lines 114) in array 110 connect pixel sensors 120 to control circuits such as row control block 130 and column control block 140 that are outside array 110. Generally, a selection signal can be asserted on one of row line 112 to select a row of pixel sensors 110 for reading via column lines 114. FIG. 1 shows only row lines 112 and column lines 114 connected to pixel sensors 120, but more generally, the circuitry in each pixel sensor 120 also connects to additional control lines (not shown).
Capturing an image with CMOS image sensor 100 generally includes a reset operation, an integration operation, and a readout operation. The reset operation resets nodes of the photodiodes in pixel sensors 120 to a reference voltage level. After the photodiode node voltages are reset, the integration operation partially discharges (or charges) the photodiode nodes via currents that flow through the photodiodes. The current through each photodiode depends on the intensity of the incident light on the photodiode, so that the voltage on the photodiode node in a pixel sensor 120 at the end of the integration operation indicates an integral of the intensity of the incident light on that pixel sensor 120 during the integration operation. The readout operation samples or measures the voltage on photodiode nodes, and those voltages can be converted to digital pixel values.
Signal noise can be a significant problem in CMOS image sensor 100, particularly during the reset operations. Ideally, a reset operation always sets the photodiode node of a pixel sensor to the same reference voltage level. If a particular pixel sensor 120 is charged to different levels during different reset operations, the pixel values readout from the pixel sensor will be inconsistent from one image to the next, leading to poor image quality.
FIG. 2 is a circuit diagram of a conventional pixel sensor 200 that is designed to provide low noise levels during reset operations. Pixel sensor 200 includes a photodiode 210, an amplifier 220, and NMOS transistors 230, 240, 250, 260, and 270. A reset operation in pixel sensor 200 includes asserting a preset signal Vpr that turns on transistor 230 to pull down a voltage Vpd on the photodiode node of pixel sensor 200. Transistor 230 is then turned off, and a signal Vg is asserted to turn on transistor 240, which connects the output of amplifier 220 to the gate of transistor 250 and completes a feedback loop for resetting of photodiode voltage Vpd. In particular, current through transistor 250 charges the photodiode node until amplifier 220 determines that voltage Vpd, which is applied to a negative input of amplifier 220, is equal to a reference voltage Vr that is applied to a positive input of amplifier 220. Amplifier 220 then shuts off transistor 250. The reset operation thus dependably charges photodiode voltage Vpd to the level of reference voltage Vr.
Transistors 240 and 250 are off during image integration to disable the feedback loop while current through photodiode 210 changes photodiode voltage Vpd. After integration, the readout operation asserts a signal WORD on the word line 112 that is coupled to pixel sensor 200, thereby turning on transistor 270. The bit line 114 connected to pixel sensor 200 is then pulled up via a current through transistor 260, which has a gate at photodiode voltage Vpd, permitting measurement of photodiode voltage Vpd through the effect on bit line 114. U.S. Pat. No. 6,424.375, entitled “Low Noise Active Reset Readout for Image Sensors” further describes operation of pixel sensors similar to pixel sensor 200.
Pixel sensor 200 has some significant drawbacks. In particular, pixel sensor 200 has an NMOS transistor 240 in the control line for the gate of NMOS transistor 250, which pulls up photodiode voltage Vpd during the reset operation. Accordingly, the upper limit of photodiode voltage Vpd must accommodate the threshold voltage drops of two NMOS transistors, which limits the dynamic range of voltage Vpd. Pixel sensor 200 is also relatively complex requiring at least six transistors and seven independent control or voltage supply lines. The circuit area required for these transistors and lines reduces the available area for photodiodes 210. As a result, the sensor array has a lower fill factor and a corresponding loss of light sensitivity.
In view of the drawbacks of existing CMOS image sensors, pixel sensors are sought that contain fewer transistors and control lines while still implementing low-noise reset operations.